As well-known in the art, a class-D amplifier converts an input signal into a pulse width modulation (hereinafter referred to as a PWM) signal having constant amplitude through the power amplification of the input signal. For example, the class-D amplifier is used for power amplification of an audio signal. Such a class-D amplifier operates as binary switches and thus can drastically lessen the loss of a transistor. Further, the class-D amplifier has higher efficiency than a linear amplifier regardless of whether the amplitude of an input signal is large or small.
FIG. 1 is a circuit diagram of a conventional class-D amplifier. As illustrated in FIG. 1, the conventional class-D amplifier includes an operational amplifying unit 10 for operationally amplifying a combined signal obtained by combining a positive input signal VINP and a first feedback signal and a combined signal obtained by combining a negative input signal VINN and a second feedback signal; a positive PWM unit 20 for producing a positive PWM signal VOUTN by comparing a positive output signal from the operational amplifying unit 10 with a reference signal VREF; and a negative PWM unit 30 for producing a negative PWM signal VOUTN by comparing a negative output signal from the operational amplifying unit 10 with the reference signal VREF. The positive PWM signal VOUTP is provided as the first feedback signal to the operational amplifying unit 10, and the negative PWM signal VOUTN is provided as the second feedback signal to the operational amplifier 10.
The PWM signals VOUTP and VOUTN from this class-D amplifier pass through a low-pass filter including a coil, a capacitor, and the like and employed to drive a load of a loudspeaker and the like.
FIGS. 2A to 2C are views showing an example of an output signal and an input signal from the conventional class-D amplifier, in which FIG. 2A depicts an input signal to the class-D amplifier, FIG. 2B depicts an ideal output signal from the class-D amplifier, and FIG. 2C depicts an actual output signal from the class-D amplifier.
As illustrated in FIGS. 2A to 2C, in view of the output signals from the class-D amplifier, it is ideal for the positive PWM signal VOUTP and the negative PWM signal VOUTN to be symmetrical, i.e., fully differential, as illustrated in FIG. 2B. As illustrated in FIG. 2C, however, the positive PWM signal VOUTP and the negative PWM signal VOUTN represent asymmetrical property in some sections within a pulse period.
The positive PWM signal VOUTP and the negative PWM signal VOUTN having such asymmetrical property are provided as feedback signals to the operational amplifying unit as shown in FIG. 2A. As a result, the amplitude of an input signal to the operational amplifier varies to a relatively large extent, and a DC loop gain and the gain-bandwidth product of the loop gain vary. Moreover, the common mode rejection properties of the operational amplifier may become worse owing to the above.
In order to solve the problems caused by the feedback signal having such asymmetrical property, it may be considered to separately employ a positive operational amplifier for receiving a positive feedback signal and a negative operational amplifier for receiving a negative feedback signal. This potential solution, however, leads to additional power consumption and an increase in size of the amplifier.